This invention relates to a process for desulfurization of molten metals including injecting a desulfurization reagent into said molten metal, said desulfurization reagent comprising magnesium nitride.
Desulfurization of molten metal by magnesium is well known, for example, in the steel manufacturing process. Generally, magnesium is introduced into the molten hot metal by any of a variety of means, for example, a desulfurization reagent may be added during steel manufacturing through injection lances. Upon injection, the magnesium melts, then vaporizes, and then the vapors dissolve into the molten hot metal. The dissolved magnesium reacts with sulfur present in the molten hot metal and forms magnesium sulfide, an insoluble compound. This insoluble compound has a density less than the density of molten hot metal and, thus, floats to the top of the molten metal and then mixes with the slag that is also floating on top of the liquid metal. The sulfur is removed by removal of the slag layer in a subsequent step. It is also well known that magnesium when injected into steel causes a violent reaction, for example, the addition of magnesium in particulate form, the violent reaction may take place in the form of bubbling, splattering, or the like. Also, finely ground particulate dust is difficult to meter in injection processes and such finely ground dust injectables create a hazard in handling. For example, finely ground particulate when exposed to high temperatures and oxygen may produce an explosion. Such may be the case with handling finely ground magnesium granules injected into a molten hot metal, such as molten steel, normally at process temperatures of up to about 1800 degrees centigrade.
Other known magnesium based injectable materials used as desulfurization reagents include magnesium granules with a surface coating of a second material. For example, U.S. Pat. No. 4,331,711 discloses magnesium or alloys of magnesium particles coated with a salt such as a halide of Na, K, Li, Mg, Ca, Ba, Mn or Sr or mixtures of these salts.
U.S. Pat. No. 4,398,947 discloses a desulfurization reagent including magnesium granules containing a coating of an anti-caking agent consisting of stearates of Mg, Ca and Al.
U.S. Pat. No. 4,401,465 discloses a desulfurization reagent including substantially nonhygroscopic flux coated magnesium granules containing a coating of fluoride-containing salt such as alkali and alkaline earth metal fluorides and fluorborates.
Mixtures of powders of materials such as magnesium and calcium or lime are also known desulfurization reagents. In addition, U.S. Pat. No. 4,705,561 discloses a composite material of a magnesium and calcium oxide useful as a desulfurization reagent.
Another useful material as a desulfurization reagent is disclosed in U.S. Pat. No. 4,708,737 which includes magnesium or aluminum impregnated into an alkaline earth compound.
It would be desirable to provide a magnesium based hot metal desulfurization reagent which is a single reagent, i.e., not a mixture or composite because mixtures may tend to separate. It is also desirable to provide a magnesium based hot metal desulfurization reagent which is easy and safe to handle, has a low injection violence and has a high desulfurization efficiency.